(366f) A Sorption-Induced Swelling Model for Sorption Hysteresis in Glassy Polyurethane Foam

Investigating the sorption hysteresis on glassy polymers, such as rigid polyurethanes, where steady-state sorbed concentration depends on whether the material is sorbing or outgassing, is critical for understanding their sink or source behavior in enclosed environments, efficacy as moisture sealants, and properties as separation membranes in a range of humidity conditions [1]. Emerging research attributes sorption hysteresis on glassy polymers to the opening of previously inaccessible sorption sites due to sorption-induced swelling [2]. Once opened, these sites remain accessible until the material is nearly dried, resulting in hysteresis due to the different concentrations of available sites during sorption and desorption [2]. In this work, we demonstrate that hysteretic moisture sorption behavior on PU foam is plausibly explained by the increase in available amine/carbonyl hydrogen bonding sites [3] by sorption-induced swelling. A model coupling chemical (sorption) and mechanical (swelling) behavior is derived to describe sorption hysteresis in PU foam.

Methods

The moisture uptake on glassy PU foam is assessed by dynamic vapor sorption at relative humidities from 0-95% at 298-353 K. Hygroscopic and thermal swelling were assessed by measuring the linear strain at humidities from 0-80% from 303-333 K on a thermomechanical analyzer equipped with a humidity generator.

Results

The steady-state sorbed moisture content during sorption and desorption of water on the glassy PU foam at 298 and 353 K is reported in Figure 1a and 1b. To describe the hysteretic sorption behavior on PU foam we derive a coupled chemo-mechanical model based on the following assumptions: (i) volumetric strain increases linearly with increasing moisture content (m_w/m_p) and temperature (T) (eq. 1), (ii) the number of accessible sites during sorption (L_s) increases linearly with increasing volumetric strain (ε_v) (eq. 1), (iii) multilayer sorption at each site is described by the Guggenheim-Anderson-de Boer (GAB) model (eq. 2) [4], and (iv) the number of sites during desorption (L_d) is constant and equal to the maximum number of sites that were rendered available during sorption (L_d=Ls(ε_v,max)). In eqs. 1 and 2, α and β are experimentally assessed thermal and hygroscopic expansion coefficients, θ is the average number of water molecules sorbed per site, a_w is the activity of water, and b and ω are the equilibrium constants for monolayer and multilayer sorption, respectively. The fits are reported as solid lines in Figure 1a and 1b for 298 and 353 K, and a parity plot comparing model prediction to experimental data at all temperatures is shown in Figure 1c. Parameter estimates are reported in Figure 1d. This model gives an excellent fit to experimental data, demonstrating the utility of this swelling-based model and the plausibility of sorption-induced swelling as the mechanism for hysteresis in PU foam.

Significance

A model coupling moisture uptake and volumetric strain is derived to describe the hysteretic sorption-desorption behavior in glassy polymers. This model is successfully applied to water uptake on a glassy polyurethane foam, demonstrating that sorption-induced swelling is a plausible mechanism for sorption hysteresis in this material.

Acknowledgement

This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.

References

[1] Minelli, M.; Sarti, G. C. 110th Anniversary: Gas and Vapor Sorption in Glassy Polymeric Membranes—Critical Review of Different Physical and Mathematical Models. Ind. Eng. Chem. Res. 2020, 59 (1), 341–365.

[2] Chen, M.; Coasne, B.; Guyer, R.; Derome, D.; Carmeliet, J. Role of Hydrogen Bonding in Hysteresis Observed in Sorption-Induced Swelling of Soft Nanoporous Polymers. Nat. Commun. 2018, 9 (1), 3507.

[3] Yu, Y.-J.; Hearon, K.; Wilson, T. S.; Maitland, D. J. The Effect of Moisture Absorption on the Physical Properties of Polyurethane Shape Memory Polymer Foams. Smart Mater. Struct. 2011, 20 (8), 085010.

[4] Brunauer, S.; Emmett, P. H.; Teller, E. Adsorption of Gases in Multimolecular Layers. J. Am. Chem. Soc. 1938, 60 (2), 309–319